Method and apparatus for performing handover in c-ran system

ABSTRACT

A method and apparatus for performing handover by a user equipment (UE) in a C-RAN system is disclosed. The method for performing handover by a user equipment (UE) in a Cloud Radio Access Network (C-RAN) includes: measuring at least one candidate remote radio head (RRH); transmitting feedback information caused by the measurement to a primary RRH; and receiving information regarding a changed primary RRH according to the measurement result received from the primary RRH, wherein the measurement is performed to discriminate each RRH on the basis of a channel state information-reference signal (CSI-RS) antenna port discriminated per the at least one candidate RRH.

TECHNICAL FIELD

The present invention relates to wireless communication, and more particularly to a method and apparatus for performing handover in a Cloud Radio Access Network (C-RAN) system.

BACKGROUND ART

The 30-years history of mobile communication commercialization on the basis of AMPS (the first-generation analog mobile communication systems) has caused a rapid change in society, such that mobile communication technologies have rapidly come into widespread use and been commercialized throughout the world. Specifically, mobile communication networks have generated a variety of changes throughout modern society in recent times, and have also rapidly developed.

Expansion of third-generation communication system has been considerably delayed due to irregular development of mobile communication and mobile computing, and fourth-generation communication systems have been rapidly developed in response to the computing environment rapidly changing from personal computers (PCs) such as desktop computers and laptops to personal information devices such as smartphones and tablets. Particularly, the recent development of cloud computing environments requires an organic combination of higher-level communication/computing technologies, such that the above-mentioned tendency will be accelerated.

In recent times, as LTE-Advanced and WiMAX-Advanced are approved as fourth-generation IMT-Advanced standards by the ITU, many developers and companies are conducting intensive research into fifth-generation mobile communication systems. For example, ITU-R WP5D taking charge of IMT serving as one of the international mobile communication standards has held a local Workshop entitled “IMT for the Next Decade” in 2011, such that the ITU-R WP5D are actively carrying out various activities arousing user interest in fifth-generation communication. Besides, WWRF has investigated requirements and versions of the NG-Wireless system and a first report has been published by the WWRF.

Since the communication service environment has rapidly changed over the last decade, it is very difficult for users to predict how much the communication service environment will change in the next decade. Recently, current communication standard organizations such as ITU-R and WWRF have considered the following change factors for 5 G communication up to the year 2020.

-   -   Development of Multimedia Service based on high-quality video         service.     -   Provision of differentiated User eXperience (UX) through         personalization service:     -   Provision of services suited to personal interest, situation,         and equipment.     -   Communication environment change from device-oriented         communication environment to user-oriented communication         environment: A user owns a plurality of communication devices,         provision of user-oriented services is requested. For example,         content charging, provision of seamless mobility between         heterogeneous devices and security service between heterogeneous         devices may be the user-based services.     -   Extension of M2M service: Increasing traffic through increase in         the number of M2M devices and provision of new M2M-based         services are requested.     -   Low Variation of Mobile Cloud Computing environment: all         computing environments are coupled to the network, such that         mobile cloud computing is provided through provision of         lower-latency and higher-performance communication environments.         The following important requirements of 5 G communication         systems have been discussed according to the changing service         environment.

1. Increase of Bandwidth/Transfer Rate

According to various research reports, it is expected that the number of mobile devices and the amount of traffic will rapidly increase over the next decade. In case of mobile devices, whereas an increase in population is not large, it is expected that low-end UEs will be replaced with Internet UEs such as smartphones or tablets and the number of connected UEs such as M2M will rapidly increase.

According to a report from Cisco, the amount of overall mobile traffic has increased by about six fold from 2008 to 2010, and will increase 26 fold by 2015 year such that monthly mobile traffic will be 6.3 EB per month. UMTS has predicted that the overall amount of mobile traffic will change from an annual of 3.8 EB in 2010 to 127 EB (corresponding to about 33 times the annual of 3.8 EB) by 2020 by referring to IDATE documents. For some years from 2010 to 2020, it is expected that the number of mobile UEs will be increased about two fold such that the number of mobile UEs will be increased from 5 billion to about 10 billion.

It can be recognized that a first object of the 5G mobile communication system is to increase a transfer rate on the basis of the above-mentioned tendency. Although various methods can be used to increase transfer rate, a first method is to find/use an additional bandwidth that is not presently being used.

Although current mobile communication systems mainly operate at 3 GHz or less, many developers and companies are conducting intensive research into higher bands due to bandwidth limitations. Specifically, various methods for guaranteeing system performance in response to frequency characteristics changed within a high frequency band of 2-6 GHz band have been intensively researched by many developers and companies.

2. Provision of Uniform Service Quality

4G communication implements a maximum transfer rate of 1 Gbps such that it has been greatly improved in terms of a transfer rate. However, the 4 G communication system has serious unbalance in service quality between a cell edge region and normal cell regions because a difference between spectrum efficiency of the cell edge region and average spectrum efficiency of a cell is 30 times or more. Due to the motto “Provision of Desired Service from anywhere at any time” of the 5 G communication system, improvement of an unbalanced service quality is of importance to the 5G communication system.

In association with the above-mentioned description, although many developers and companies are conducting intensive research into performance improvement in the cell edge region through WiFi offloading, addition of an auxiliary cell such as Femto BS, eICIC (Enhanced Inter-cell Interference Coordination) and CoMP (Coordinated Multi-Point) technologies for use in the 4G system, there is a need to provide a higher-level uniform service.

3. User-Oriented Organic Interoperation

Although the 4G system has implemented interoperation between devices on the basis of user handling or predetermined policy, the next-generation communication system requires diversification of UEs, organic interoperation of several devices, and various technologies for providing the same service level in various communication environments. There are needed various technologies in which communication technologies such as cellular or WLAN are organically interoperable without using complicated processes and user intervention is minimized through seamless services.

Measurement establishment for enabling a UE to perform handover in a cellular network has already been proposed. However, when the UE performs handover in the C-RAN system, a method for performing UE measurement, a method for classifying nodes in the C-RAN system during the UE measurement, and a method for enabling the UE to perform handover through such measurement have yet to be proposed.

DISCLOSURE OF INVENTION Technical Problem

Accordingly, the present invention is directed to a method and apparatus for enabling a user equipment (UE) to perform handover in a Cloud Radio Access Network (C-RAN) system that substantially obviates one or more problems due to limitations and disadvantages of the related art.

An other object of the present invention is to provide a method for enabling a user equipment (UE) to perform handover in a Cloud Radio Access Network (C-RAN) system.

Another object of the present invention is to provide a user equipment (UE) for performing handover in a C-RAN system.

It is to be understood that technical objects to be achieved by the present invention are not limited to the aforementioned technical objects and other technical objects which are not mentioned herein will be apparent from the following description to one of ordinary skill in the art to which the present invention pertains.

Solution to Problem

The object of the present invention can be achieved by providing a method for performing a handover by a user equipment (UE) in a Cloud Radio Access Network (C-RAN), the method including: measuring at least one candidate remote radio head (RRH); transmitting feedback information caused by the measurement to a primary RRH; and receiving information regarding a changed primary RRH from the primary RRH according to a result of the measurement, wherein the measurement is performed to discriminate each RRH on the basis of a channel state information-reference signal (CSI-RS) antenna port discriminated per the at least one candidate RRH.

The method may further include: receiving list information of the at least one candidate RRH from a base station (BS) or the primary RRH. The at least one candidate RRH may include the primary RRH and an RRH communicating with the UE. The changed primary RRH may correspond to an RRH having a highest signal intensity from among the measurement result. The method may further include: transmitting a RACH to the changed primary RRH through RRH dedicated RACH resources. The method may further include: receiving information regarding a CSI-RS antenna port discriminated per RRH from a base station (BS).

In another aspect of the present invention, a user equipment (UE) for performing a handover in a Cloud Radio Access Network (C-RAN) includes: a receiver; a transmitter; and a processor, wherein the processor measures at least one candidate remote radio head (RRH), controls the transmitter to transmit feedback information caused by the measurement to a primary RRH, and controls the receiver to receive information regarding a changed primary RRH from the primary RRH according to a result of the measurement, and the processor performs the measurement to discriminate each RRH on the basis of a channel state information-reference signal (CSI-RS) antenna port discriminated per the at least one candidate RRH.

The processor may receive list information of the at least one candidate RRH from a base station (BS) or the primary RRH. The at least one candidate RRH may include the primary RRH and an RRH communicating with the UE. The processor may control the transmitter to transmit a RACH to the changed primary RRH through RRH dedicated RACH resources. The processor may control the receiver to receive information regarding a CSI-RS antenna port discriminated per RRH from a base station (BS). The changed primary RRH may correspond to an RRH having the highest signal intensity from among the measurement result.

Advantageous Effects of Invention

As is apparent from the above description, the UE performs efficient measurement for handover in the C-RAN system, and properly selects a primary RRH or a serving RRH so as to performs communication, resulting in an increase in communication performance.

It will be appreciated by persons skilled in the art that the effects that can be achieved with the present invention are not limited to what has been particularly described hereinabove and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention.

FIG. 1 is a block diagram illustrating a base station (BS) and a user equipment (UE) for use in a wireless communication system.

FIG. 2 is a conceptual diagram illustrating modification of BS/RAN structures through an RRH concept and RRH.

FIG. 3 is a conceptual diagram illustrating a cloud network based on C-RAN.

FIG. 4 is a conceptual diagram illustrating a user-oriented cell.

FIG. 5 is a conceptual diagram illustrating an RRH and virtual BS (VBS) shared scenario in C-RAN.

FIG. 6 is a conceptual diagram illustrating a model of multiple control layers.

FIG. 7 is a conceptual diagram illustrating a next-generation network supporting situation-cognition based intelligence interoperation.

FIG. 8 is a flowchart illustrating a conventional X2 based handover procedure.

FIG. 9 is a conceptual diagram illustrating a method for supporting mobility through RRH node switching in a C-RAN according to the present invention.

FIG. 10 is a flowchart illustrating a primary RRH switching process in response to UE movement in a C-RAN according to the present invention.

FIGS. 11A and 11B illustrate a Cell Reference Signal Received Power (RSRP) changing with UE movement.

FIGS. 12A and 12B illustrate a cell RSRP changing with UE movement.

FIG. 13 shows a table showing the connected RRH MAC control element.

FIG. 14 shows a table illustrating primary RRH switching MAC control

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The detailed description, which will be given below with reference to the accompanying drawings, is intended to explain exemplary embodiments of the present invention, rather than to show the only embodiments that can be implemented according to the present invention. The following detailed description includes specific details in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without such specific details. For example, the following description will be given centering upon a mobile communication system serving as a 3GPP LTE system, but the present invention is not limited thereto and the remaining parts of the present invention other than unique characteristics of the 3GPP LTE system are applicable to other mobile communication systems.

In some cases, in order to prevent ambiguity of the concepts of the present invention, conventional devices or apparatuses well known to those skilled in the art will be omitted and be denoted in the form of a block diagram on the basis of important functions of the present invention. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

In the following description, a terminal may refer to a mobile or fixed user equipment (UE), for example, a user equipment (UE), a mobile station (MS) and the like. Also, the base station (BS) may refer to an arbitrary node of a network end which communicates with the above terminal, and may include an eNode B (eNB), a Node B (Node-B), an access point (AP) and the like. Although the embodiments of the present invention are disclosed on the basis of IEEE 802.16 for convenience of description, contents of the present invention can also be applied to other communication systems.

In a mobile communication system, the UE may receive information from the base station (BS) via a downlink, and may transmit information via an uplink. The information that is transmitted and received to and from the UE includes data and a variety of control information. A variety of physical channels are used according to categories of transmission (Tx) and reception (Rx) information of the UE.

The following embodiments of the present invention can be applied to a variety of wireless access technologies, for example, CDMA (Code Division Multiple Access), FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), OFDMA (Orthogonal Frequency Division Multiple Access), SC-FDMA (Single Carrier Frequency Division Multiple Access), and the like. CDMA may be embodied through wireless (or radio) technology such as UTRA (Universal Terrestrial Radio Access) or CDMA2000. TDMA may be embodied through wireless (or radio) technology such as GSM (Global System for Mobile communication)/GPRS (General Packet Radio Service)/EDGE (Enhanced Data Rates for GSM Evolution). OFDMA may be embodied through wireless (or radio) technology such as Institute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA (Evolved UTRA). UTRA is a part of UMTS (Universal Mobile Telecommunications System). 3GPP (3rd Generation Partnership Project) LTE (long term evolution) is a part of E-UMTS (Evolved UMTS), which uses E-UTRA. 3GPP LTE employs OFDMA in downlink and employs SC-FDMA in uplink. LTE-Advanced (LTE-A) is an evolved version of 3GPP LTE.

It should be noted that specific terms disclosed in the present invention are proposed for convenience of description and better understanding of the present invention, and the use of these specific terms may be changed to other formats within the technical scope or spirit of the present invention.

FIG. 1 is a block diagram illustrating a base station (BS) 105 and a user equipment (UE) 110 for use in a wireless communication system 100 according to the present invention.

Although FIG. 1 shows one UE 105 and one UE 110 (including a D2D UE) for brief description of the wireless communication system 100, it should be noted that the wireless communication system 100 may further include one or more BSs and/or one or more UEs.

Referring to FIG. 1, the BS 105 may include a transmission (Tx) data processor 115, a symbol modulator 120, a transmitter 125, a transmission/reception antenna 130, a processor 180, a memory 185, a receiver 190, a symbol demodulator 195, and a reception (Rx) data processor 197. The UE 110 may include a Tx data processor 165, a symbol modulator 170, a transmitter 175, a transmission/reception antenna 135, a processor 155, a memory 160, a receiver 140, a symbol demodulator 155, and an Rx data processor 150. In FIG. 1, although one antenna 130 is used for the BS 105 and one antenna 135 is used for the UE 110, each of the BS 105 and the UE 110 may also include a plurality of antennas as necessary. Therefore, the BS 105 and the UE 110 according to the present invention support a Multiple Input Multiple Output (MIMO) system. The BS 105 according to the present invention can support both a Single User-MIMO (SU-MIMO) scheme and a Multi User-MIMO (MU-MIMO) scheme.

In downlink, the Tx data processor 115 receives traffic data, formats the received traffic data, codes the formatted traffic data, interleaves the coded traffic data, and modulates the interleaved data (or performs symbol mapping upon the interleaved data), such that it provides modulation symbols (i.e., data symbols). The symbol modulator 120 receives and processes the data symbols and pilot symbols, such that it provides a stream of symbols.

The symbol modulator 120 multiplexes data and pilot symbols, and transmits the multiplexed data and pilot symbols to the transmitter 125. In this case, each transmission (Tx) symbol may be a data symbol, a pilot symbol, or a value of a zero signal (null signal). In each symbol period, pilot symbols may be successively transmitted during each symbol period. The pilot symbols may be an FDM symbol, an OFDM symbol, a Time Division Multiplexing (TDM) symbol, or a Code Division Multiplexing (CDM) symbol.

The transmitter 125 receives a stream of symbols, converts the received symbols into one or more analog signals, and additionally adjusts the one or more analog signals (e.g., amplification, filtering, and frequency upconversion of the analog signals), such that it generates a downlink signal appropriate for data transmission through an RF channel. Subsequently, the downlink signal is transmitted to the UE through the antenna 130.

Configuration of the UE 110 will hereinafter be described in detail. The antenna 135 of the UE 110 receives a DL signal from the BS 105, and transmits the DL signal to the receiver 140. The receiver 140 performs adjustment (e.g., filtering, amplification, and frequency downconversion) of the received DL signal, and digitizes the adjusted signal to obtain samples. The symbol demodulator 145 demodulates the received pilot symbols, and provides the demodulated result to the processor 155 to perform channel estimation.

The symbol demodulator 145 receives a frequency response estimation value for downlink from the processor 155, demodulates the received data symbols, obtains data symbol estimation values (indicating estimation values of the transmitted data symbols), and provides the data symbol estimation values to the Rx data processor 150. The Rx data processor 150 performs demodulation (i.e., symbol-demapping) of data symbol estimation values, deinterleaves the demodulated result, decodes the deinterleaved result, and recovers the transmitted traffic data.

The processing of the symbol demodulator 145 and the Rx data processor 150 is complementary to that of the symbol modulator 120 and the Tx data processor 115 in the BS 205.

The Tx data processor 165 of the UE 110 processes traffic data in uplink, and provides data symbols. The symbol modulator 170 receives and multiplexes data symbols, and modulates the multiplexed data symbols, such that it can provide a stream of symbols to the transmitter 175. The transmitter 175 receives and processes the stream of symbols to generate an uplink (UL) signal, and the UL signal is transmitted to the BS 105 through the antenna 135.

The BS 105 receives the UL signal from the UE 110 through the antenna 130. The receiver processes the received UL signal to obtain samples. Subsequently, the symbol demodulator 195 processes the symbols, and provides pilot symbols and data symbol estimation values received via uplink. The Rx data processor 197 processes the data symbol estimation value, and recovers traffic data received from the UE 110.

A processor 155 or 180 of the UE 110 or the BS 105 commands or indicates operations of the UE 110 or the BS 105. For example, the processor 155 or 180 of the UE 110 or the BS 105 controls, adjusts, and manages operations of the UE 210 or the BS 105. Each processor 155 or 180 may be connected to a memory unit 160 or 185 for storing program code and data. The memory 160 or 185 is connected to the processor 155 or 180, such that it can store the operating system, applications, and general files.

While the UE processor 155 enables the UE 110 to receive signals and can process other signals and data, and the BS processor 180 enables the BS 105 to transmit signals and can process other signals and data, the processors 155 and 180 will not be specially mentioned in the following description. Although the processors 155 and 180 are not specially mentioned in the following description, it should be noted that the processors 155 and 180 can process not only data transmission/reception functions but also other operations such as data processing and control.

The processor 155 or 180 may also be referred to as a controller, a microcontroller), a microprocessor, a microcomputer, etc. In the meantime, the processor 155 or 180 may be implemented by various means, for example, hardware, firmware, software, or a combination thereof. In a hardware configuration, methods according to the embodiments of the present invention may be implemented by the processor 155 or 180, for example, one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, etc.

In a firmware or software configuration, methods according to the embodiments of the present invention may be implemented in the form of modules, procedures, functions, etc. which perform the above-described functions or operations. Firmware or software implemented in the present invention may be contained in the processor 155 or 180 or the memory unit 160 or 185, such that it can be driven by the processor 155 or 180.

Radio interface protocol layers among the UE 110, the BS 105, and a wireless communication system (i.e., network) can be classified into a first layer (L1 layer), a second layer (L2 layer) and a third layer (L3 layer) on the basis of the lower three layers of the Open System Interconnection (OSI) reference model widely known in communication systems. A physical layer belonging to the first layer (L1) provides an information transfer service through a physical channel. A Radio Resource Control (RRC) layer belonging to the third layer (L3) controls radio resources between the UE and the network. The UE 110 and the BS 105 may exchange RRC messages with each other through the wireless communication network and the RRC layer.

While the UE processor 155 enables the UE 110 to receive signals and can process other signals and data, and the BS processor 180 enables the BS 105 to transmit signals and can process other signals and data, the processors 155 and 180 will not be specially mentioned in the following description. Although the processors 155 and 180 are not specially mentioned in the following description, it should be noted that the processors 155 and 180 can process not only data transmission/reception functions but also other operations such as data processing and control.

5G communication network technology is largely classified into a radio access network field and a core network field. Generally, two technical fields are present in the radio access network field. First, a centralized access network may be used through introduction of a network cloud. Three core technologies for implementing the network cloud are Remote Radio Head (RRH)/Coordinated Multi-Point (CoMP) technology, software modem technology, and cloud computing technology.

The most important core technology for implementing the network cloud in the radio access network field is the introduction of RRH. Although RRH is very important in terms of radio transmission, the RRH may greatly change a radio access network structure.

FIG. 2 is a conceptual diagram illustrating modification of BS/RAN structures through RRH concept and RRH.

Although RRH has been developed as one of optical repeaters, RRH has recently been used as a core element to implement a centralized base station. The most important core technology for implementing the network cloud in the radio access network field is the introduction of RRH. RRH is very important in terms of radio transmission, and may greatly change the radio access network structure. With the introduction of RRH, a conventional base station is no longer physically distributed due to physical distribution of Radio Frequency Units (RFUs) and Baseband Units (BBUs). Recently, a cloud access network is interoperable with several hundreds of RRHs through only one device so as to implement network operation/management, resulting in formation of a cell different from a typical cell.

Although communication systems to the 4 G communication system have defined all radio access operations on the basis of a cell, a new cell concept needs to be established through the above structural change. Presently, 3GPP has intensively proposed various scenarios on the condition that RRH and a macro BS are present through Release 11 CoMP (Coordinated Multi-Point) Work Item. In recent times, various research into enabling several cells to share one RRH as in a Shared Antenna System (SAS) have also been proposed. In addition, a method for dynamically changing a cell region by adjusting an RRH cluster according to a situation has also been recently proposed. Due to these situations, user interest in the Cloud Radio Access Network (C-RAN) project is rapidly increasing.

FIG. 3 is a conceptual diagram illustrating a cloud network based on C-RAN.

FIG. 3 is a conceptual diagram of a C-RAN. The C-RAN may include a plurality of RRHs, a software-based virtual base station (VBS), an access control server for controlling RRHs and VBSs, and a core network cloud server (including a resource management server, an accounting/authentication server, etc.). As described above, as elements of the core network are gradually changed to the open IP network, C-RAN elements are organically interoperable with the core network elements.

Referring to FIG. 3, several RRGs are connected to a virtual base station (VBS) through an optical access device. The VBS is implemented by software, and may be implemented by various radio access technologies such as Long Term Evolution (LTE), HSPA, WiMAX/WiFi, etc., and one or more RRHs are grouped and controlled by a single VBS. While the cell region is fixed in the related art, C-RAN dynamically changes RRH clusters so that cells can be dynamically allocated. Such dynamic allocation may also be adjusted according to distribution of users present in the region. Therefore, there is a need to consider a method for removing the cell concept and constructing a cell per user.

As can be seen from FIG. 3, several RRHs are connected to the VBS through an optical access device. The VBS may be implemented by software, and may also be implemented by various radio access technologies such as LTE, HSPA, WiMAX/WiFi, etc. One or more RRHs are grouped and controlled by a single VBS. While the cell region is fixed in the related art, RRH clusters are dynamically changed in the C-RAN system so that cells can be dynamically allocated.

FIG. 4 is a conceptual diagram illustrating a user-oriented cell.

In C-RAN, RRH clusters are dynamically changed so that cells are dynamically allocated. Such dynamic allocation may also be adjusted according to distribution of users present in the region. Some developers and companies have conducted intensive research into a method for removing the cell concept and constructing a per-user cell on the basis of a current user.

FIG. 5 is a conceptual diagram illustrating an RRH and VBS shared scenario in C-RAN.

A virtual structure such as a C-RAN has proposed a new possibility in consideration of network opening and network sharing characteristics. Individual countries have executed various policies [e.g., MVNO, BS(Base Station)/AP(Access Point)] to introduce competitive elements to a communication market. For example, MVNO, BS(Base Station)/AP(Access Point), etc. If the VBS is introduced to the market, it is possible to use various service scenarios, for example, a virtual enterprise may construct a VBS under the condition that an interface with an RRH is maintained, and the same radio resources may be shared by a plurality of VBSs.

In FIG. 5, enterprises A, B and C may share the same RRH pool so as to provide services. Especially, the enterprise B and the enterprise C may provide different services using the same VBS. The VBS enterprise may charge a usage fee to each of the enterprises B and C according to the amount of radio resources used. Each enterprise applies a unique resource allocation policy to predetermined radio resources so as to support a subscriber.

Services for selling frequency resources in real time have recently been activated in the United States of America, and a new frequency-associated business capable of leasing some frequency resources may also be possible in the VBS environment.

The second important flow of the radio access network field is to strengthen a distributed layer. Although the centralized and central processing function of the radio access network has been strengthened through C-RAN, there is a limit to the capabilities of the VBS and access server of the C-RAN. Specifically, assuming that a base station (BS) such as a femto BS is connected through a private IP network, it is very difficult to manage the BS in real time. In addition, it is very difficult for the network to control all operations of Device-to-Device (D2D) communication. As a result, while a common access network based on C-RAN evolves in the form of central control processing, localized communication such as femto BS and D2D communication may have a distributed control structure.

FIG. 6 is a conceptual diagram illustrating a model of multiple control layers.

Referring to FIG. 6, a model of the multiple control layers (hereinafter referred to as a multi-control layer model) includes a central control layer controlled through a cloud access network and a distributed control layer. The distributed control layer is controlled by each communication entity whereas it is partially controlled by the central control layer. Studies into interference control for coexistence between layers are of importance to the multi-control layer model.

In the core network aspect, it is expected that the core network will evolve into an All IP-based Open Network in view of 4G system continuity. In recent times, various services have a tendency to be changed from network enterprise based services such as IMS to Web/application layer based services. The above-mentioned fact can be confirmed through a comparison between the enterprise based Rich Communication Suite (RCS) and the service enterprise based Over The Top (OTT) service. Although it is difficult for the user to make a hasty conclusion of the confirmation result, SMS-, MMS-, and IMS-based RCS services of communication companies will be replaced with OTT services such as Kakao Talk and Skype.

Specifically, the aforementioned tendency will be accelerated through activation of the next-generation Web standards such as HLML5 and mobile cloud services. In response to change of service fields, the core network function will focus on provision of IP transport to a radio network, and it is expected that the core network will be developed from the legacy voice-service-based hierarchical network to a more horizontal IP network. As a result, many conventional network elements are simplified in structure, and network elements are gradually changed from the large-sized server-dependent structure to a structure implemented through a plurality of core network cloud servers, such that the above-mentioned network development can be achieved through lower CAPEX/OPEX.

The latent principal issue of the core network other than service provision is provision of IP flow mobility. Heterogeneous network support technology such as Multi-Access PDN connectivity (MAPCON) or IP Flow Mobility (IFOM) has been standardized in 3GPP SA2. Specifically, offloading through interaction with a WLAN is of importance to 3GPP SA2. In contrast, with the introduction of C-RAN, an interaction structure between heterogeneous networks is simplified so that it is expected that IP-based mobility control will also be greatly simplified. Assuming that the related art has proposed services through two different PDNs, the above-mentioned services can be implemented by only one method through interaction between the C-RAN access control server and the core-network integrated mobility control server, irrespective of the radio access scheme employed. In addition, an Access Node Discovery and Selection Function (ANDSF) being introduced for the offloading policy may be easily contained in the core network (CN) mobility control server region and then simplified.

In recent times, as information technology devices become highly intelligent, management elements that have been confined to UE radio characteristics, traffic types, accounting, authentication, etc. will evolve in more various ways. A smartphone of the user collects basic information, personal location information, use and movement pattern, user interest, biomedical or vital information, etc., and the network will provide user-oriented services using the above-mentioned information. For this purpose, it is expected that a function for collecting/processing large volumes of data will be added to the core network and then strengthened. While the legacy core network provides necessary services on the basis of the enterprise policy, a method for collecting, analyzing, and processing large volumes of data so as to provide the user-oriented service will be intensively discussed and studied in the next-generation communication network.

Specifically, interaction between heterogeneous networks is facilitated, such that providing optimum radio access suitable for a current situation through analysis of the corresponding big data is of importance to the network enterprise and users.

The importance of research into associated technical fields will gradually increase.

FIG. 7 is a conceptual diagram illustrating a next-generation network supporting situation-cognition based intelligence interoperation.

Referring to FIG. 7, various information collected by a UE is transferred to the access network server and the core network server, such that an optimum access environment of the UE can be controlled on the basis of the collected information.

The above-mentioned description has disclosed the network evolution for the next-generation communication system in view of the radio access network and the core network. The radio access network evolves into the cloud network such that extensibility and network flexibility will be improved. In addition, as the control region is gradually extended due to centralization, a large number of functions associated with legacy core network communication will be connected to the access network. In contrast, as the core network is gradually developed in consideration of network intelligence, it will be possible to provide an intelligent access control function and an interaction control function based on various situation information collected by a user, an access network and other environmental elements. The development direction of 5 G communication has just started to be laid out. It is expected that 5 G communication technology will be developed to a new wireless communication technology accompanied with appearance of IT technology and bio- or nano-technology.

The principal change in a Cloud RAN (C-RAN) is that an inter-cell handover (HO) concept is changed to the switching between RRH nodes. The handover (HO) procedure in the C-RAN may be different from a legacy inter-cell handover procedure due to such C-RAN change.

FIG. 8 is a flowchart illustrating a conventional X2 based handover procedure.

Referring to FIG. 8, a serving eNB (S_eNB) may provide a user equipment (UE) with measurement configuration information in step S810. If measurement of each neighbor cell is configured by such measurement configuration, and if a specific situation is satisfied, the UE begins measurement in step S820, and reports the measurement result to the serving eNB (S_eNB) under a specific situation in step S830. The serving eNB (S_eNB) may transmit a handover request to a target eNB (T_eNB) according to the reporting result in step S840. Upon receiving a response to the handover request, the target eNB (T_eNB) transmits a handover request response message including a handover (HO) command to the serving eNB (S_eNB) in step S850. Thereafter, the serving eNB (S_eNB) transmits a message including the HO command to the UE in step S860. As a result, a handover (HO) message is generated through negotiation between the serving eNB (S_eNB) and the target eNB (T_eNB). Thereafter, the UE attempts to perform random access to the target eNB (T_eNB) in step S870. Thereafter, the UE transmits an HO confirmation message to the target eNB (T_eNB) in step S880.

The above-mentioned process is classified into a S1-based handover on the basis of MME and a X2-based handover on the basis of negotiation between eNBs. For convenience of description, FIG. 8 assumes X2-based handover.

C-RAN Based RRH Switching

Handover (HO) in the C-RAN is also achieved in a similar manner as described above. Several RRHs are connected to a virtual base station (VBS) on a C-RAN, and the UE changes only the RRH without changing the eNB or BS. Basically, since the RRH situation assumes Joint Transmission(JT)/Joint Reception (JR), the UE can perform seamless handover.

FIG. 9 is a conceptual diagram illustrating a method for supporting mobility through RRH node switching in a C-RAN according to the present invention.

Referring to FIG. 9, associated RRH sets and connected RRH sets are contained in the cell. Cell configuration may be comprised of two scenarios (Scenario 1 and Scenario 2) on the basis of identifiers (IDs). In Scenario 1, ‘PHY cell id=MAC cell id’ is achieved, and the cell is identified by a synchronous channel of the BS, and RRH may be identified for each antenna node according to a CSI-RS. In Scenario 2, ‘PHY cell id !=MAC cell id’ is achieved, and the cell may be identified by a logical cell ID, and RRH may be identified by a synchronous channel.

One primary RRH has the same concept as the serving cell of the legacy cellular system. Generally, one primary RRH will be set to an RRH having the highest reception signal intensity on downlink.

1˜n (for example, 2 or 3) connected RRH(s): Connected RRH(s) may be RRH(s) participating in data transmission/reception, and may be allocated by the BS. Generally, activation/deactivation is achieved using a MAC control element (for example, a signaling header), and the UE always performs MIMO-associated measurement/report using CSI-RS or the like.

1˜m (m(4˜8)≧n) associated RRH sets: Associated RRH set is a set of RRHs configured to periodically perform monitoring according to situations, and indicates candidate RRHs to serve as the connected RRHs. The primary RRH may inform the UE of the RRH list associated with the associated RRHs. Alternatively, a specific Global CSI-RS port is allocated to each RRH, such that the UE may identify each RRH using the corresponding port. The BS performs measurement configuration in units of this divided allocation unit, and performs measurement and report according to the configuration result.

The RRH switching handover operation in the C-RAN environment is similar to the legacy handover operation. As shown in FIG. 9, a connected RRH set and an associated RRH set are present in the UE. In the present invention, the connected RRH set is an aggregate of RRHs that are capable of performing JT/JR to communicate with another party. An associated RRH set is an aggregate of RRHs, each of which includes a connected RRH and performs measurement. The associated RRH set is defined as candidate RRHs.

Measurement for such RRHs may be achieved in various ways, and a detailed description thereof will be given below. A primary RRH from among connected RRHs always communicates with the UE in the same manner as in the serving cell. As can be seen from FIG. 9, if the UE moves to another region, the primary RRH and some connected RRHs participate in communication, and one RRH from among the connected RRHs may be changed to a new primary RRH according to the UE movement. The associated RRH set may be allocated by the BS, and may be broadcast from the primary RRH to the UE.

Primary RRH Switching in C-RAN

FIG. 10 is a flowchart illustrating a primary RRH switching process in response to UE movement in a C-RAN according to the present invention.

As the UE moves to another region in the C-RAN, the UE needs to be handed over to the region, and this process corresponds to the primary RRH switching process. Referring to FIG. 10, RRH1 is a primary RRH, and the UE transmits and receives data to and from RRH1 in step S1005. In this case, it is assumed that RRH1 is a connected RRH. If the UE needs to be handed over, the primary RRH informs the UE of the list of associated RRHs indicating candidate RRHs in step S1010. The UE measures the associated RRHs according to measurement information configured by the primary RRH in step S1015. The UE transmits the measurement result as feedback information to an RRH (RRH1). Thereafter, the primary RRH transmits the feedback information to the VBS in step S1020. Cell selection/reselection is performed according to such feedback information. The UE includes an activation command of RRH2 in feedback information on the basis of the measurement result in step S1020, and transmits the resultant feedback information to the VBS.

The BS transmits a MAC signal indicating activation of RRH2 to the RRH1, and may transmit the MAC signal to the UE in step S1025. RRH1 and RRH2 are contained in the connected RRH. During activation, the UE performs additional measurement such as Channel Quality Information (CQI) and Precoding Matrix Index (PMI) and may feed back the measurement result to the primary RRH indicating RRH1, and the RRH1 may transmit the feedback result to the BS in step S1030.

Thereafter, the BS transmits a signal indicating that the primary RRH is changed from RRH1 to RRH2 to the RRH1, and may transmit the signal to the UE in step S1035. The primary RRH may be used as RRH2, and the connected RRH includes RRH1 and RRH2.

RRH2 updates the list of associated RRHs and may transmit the list of associated RRHs to the UE in step S1040. In step S1050, the UE reports measurement information (S1045) and feedback information to an RRH2 on the basis of the updated associated RRH list information as in step S1010. Therefore, the UE can perform RRH handover (or switching). In this case, feedback information may include a specific command incapable of deactivating the RRH1 on the basis of the measurement result so that the resultant feedback information can be transmitted. The BS transmits a MACK signal indicating deactivation of RRH1 to an RRH2, and the RRH2 may transmit the MACK signal to the UE in step S1055. Only the RRH2 is contained in the connected RRH.

A measurement process for cell selection/reselection (or RRH selection/reselection) and inter-cell handover (or handover between RRHs) will hereinafter be described in detail.

Cell Selection/Reselection (or RRH, Selection/Reselection)

Assuming the presence of both a cell and an RRH, the following three cell selection procedures can be used. Basically, cell selection is based on a common reference signal (CRS). Instead of cell selection, a random access channel (RACH) may be RRH-specifically used for high efficiency. The following table 1 shows various cell selection(reselection) methods. As can be seen from FIG. 10, the BS transmits a message indicating RRH2 selection to the primary RRH (RRH1), and the RRH1 may transmit the message to the UE.

Table 1

TABLE 1 Method 1 Method 2 Method 3 Cell Selection CRS based CRS based Global CSI-RS Criteria based for RRH RACH Cell common RACH resources for RRH dedicated transmission RACH RRH having the RACH resources(Cell ID highest power(RRH resources(RRH based) ID based) ID based) Primary RRH Allocated by BS The strongest RRH The strongest RRH BCH/Paging Cell Common Cell Common Reselection Cell Common Cell Common RRH specific Parameter

Referring to Table 1, when Method 1 and Method 2 are performed according to the cell selection criteria, the cell selection is based on a CRS. In case of Method 2, cell selection is performed for Global CSI-RS (Channel State Information-Reference Signal) for RRH.

For allocation of the primary RRH, the BS allocates the primary RRH in case of Method 1, and the RRH having the highest reception signal intensity is used as the primary RRH in case of Method 2 and Method 3.

In case of Method 1 in RACH transmission, the UE transmits a RACH through the cell common RACH resources allocated based on a cell ID. Meanwhile, according to Method 2, the UE determines an RRH having the highest reception power to be the primary RRH, transmits a RACH through RACH resources (RRH ID-based allocation) of an RRH having the highest reception power. In case of Method 3, the UE transmits an RACH to the RRH having the highest reception power through RRH dedicated RACH resources based on an RRH ID.

Method 1 and Method 2 are cell-commonly applied to BCH(Broadcast Channel)/paging transmission. Method 1 and Method 2 are cell-commonly applied to the reselection parameter. In case of Method 3, the reselection parameter can also be specifically established for each RRH.

Measurement for Inter-Cell Handover (or Handover Between RRHs)

Measurement in the C-RAN handover situation assumes that an RRH is allocated a global CSI-RS and the allocated result is used. Assuming that all RRHs in the same cell transmit the same CRS, individual RRHs are allocated different global CSI-RS ports and can be measured independently.

1. Handover Measurement Method

Cell RSRP (Reference Signal Received Power) can be determined to be a sum of CRSs of several RRHs. RRH RSRP may be measured using RSs (such as Global CSI-RS) separately allocated to individual RRHs. In the C-RAN handover situation, a handover result is compared with inter-cell handover in the legacy cellular network such that a new measurement entity or a new measurement allocation method must be defined.

2. Handover Trigger Condition

In Method 1, RSRP_cell (i.e., an RSRP value of the measured serving cell) of the serving cell is compared with another RSRP_cell (i.e., an RSRP value of the measured target cell). If the RSRP of the target cell is higher than the RSRP of the serving cell, triggering can be performed. However, if the numbers of per-cell RRHs are different from each other, there may occur unfairness.

In Method 2, an RSRP value (RSRP_RRH1) is compared with an RSRP value (RSRP_RRH2). Generally, Method 2 is achieved by extending the legacy cell-based concept to an RRH. However, since an RRH radius is very small in size, there is a high possibility that handover abruptly and frequently occurs. In Method 2, measurement complexity reduction or reliability must be guaranteed in response to the increasing number of RRHs to be measured.

The sum of RSRPs of N optimum reception RRHs contained in each cell is compared in Method 3. Assuming the CoMP JT/JR scheme, the sum of optimum RSRPs corresponding to the number of currently received RRHs may be defined. The above-mentioned scheme has high complexity, and has difficulty in measuring a combining gain of several RRHs.

3. New Measurement Configuration for C-RAN Handover

Items different from those of the related art must be added to measurement configuration so as to perform handover in the C-RAN environment. For example, the following items may be added to the measurement configuration.

Information for measuring an RRH of a neighbor cell is needed. Global CSI-RS associated information (typically including the list of cell IDs) may be used as information for measuring an RRH of a neighbor cell. In addition, a new triggering condition and measurement object information must be defined.

Conventionally, the UE has measured an RSRP and an RSRQ (Reference Signal Received Quality).

FIGS. 11A and 11B illustrate a Cell Reference Signal Received Power (RSRP) changing with UE movement.

FIG. 11A shows an RSRP change within one macro cell, and FIG. 11B shows an RSRP change when 19 RRHs are present in one macro cell. As can be seen from FIG. 11B, the cell RSRP value (RSRP_cell) moves to largely swing in the vicinity of RRH.

The correlation between RSRP_RRH and RSRP_cell in FIGS. 11A and 11B can be used for new purposes. If a high measurement value is obtained, this means that a small number of RRHs contained in the same cell may be located at a cell edge. If a low measurement value is obtained, this means that a large amount of RRHs are present in a peripheral region and may be located at a cell center. If the proposed value is a predetermined value or higher, this value may be used as triggering for measuring a CRS of another cell.

FIGS. 12A and 12B illustrate a cell RSRP changing with UE movement.

FIG. 12A shows a difference between RSRP_cell and RSRP_RRH in a macro cell, and FIG. 12B shows not only RSRP_cell of the macro cell but also a difference between RSRP_cell and RSRP_RRH. A difference value (RSRP_cell-RSRP_RRH) between RSRP_cell and RSRP_RRH may be used for new purposes. If the measurement value (i.e., a difference between RSRP_cell and RSRP_RRH) is high, this means that a small number of RRHs is present in the same cell and may be located at a cell edge. If the measurement value (i.e., a difference value between RSRP_cell and RSRP_RRH) is small, this means that a large number of RRHs are present in a peripheral region and may be located at a cell center. If the proposed measurement value (i.e., a difference value between RSRP_cell and RSRP_RRH) is a predetermined value or higher, this value may be used as triggering for measuring a CRS of another cell.

Measurement in C-RAN RRH Switching Situation

It is assumed that all RRHs of the same cell transmit the same CRS and respective RRHs are allocated different global CSI-RS ports and can be measured independently from each other.

1. RRH Measurement Method

It is assumed that the UE measures Global CSI-RS of an RRH to measure an RSRP of the RRH. The UE can measure associated RRHs. For this purpose, measurement configuration for each associated RRH must be configured. In this case, a new report trigger condition different from the report trigger condition according to the legacy inter-cell handover measurement must be defined and used as follows.

Definition of New Report Trigger Condition

If RSRP_RRH>T (where T is a predetermined threshold value) and ‘RSRP_RRH>a×RSRP_RRH_serving (0<a<1)’ are satisfied, the report triggering condition can also be satisfied. “RSRP_RRH>a×RSRP_RRH_serving” indicates that the RSRP ratio of a current serving RRH is equal to or higher than a predetermined value [this may be “RSRP_RRH(dB)>RSRP_RRH_serving(dB)−TH(dB)” (where TH is a predetermined threshold value)].

If a predetermined report triggering condition from among associated RRHs is satisfied, the report triggering condition is satisfied. According to the reporting result, the BS may change the connected RRH and the primary RRH.

Method for Changing Connected RRH

An associated RRH is mapped to each global CSI-RS port such that the UE can measure the associated RRH. A MAC control element denoted by a 1-bit bitmap can be generated at each global CSI-RS port. As a result, the MAC control element may include as much bitmap data as the number of global CSI-RS ports in the associated RRH. Each bitmap may include an RRH activation(1)/deactivation(0) command. In case of UE activation, additional operations associated with the corresponding global CSI-RS port can be carried out. In case of UE activation, additional measurement of CQI and PMI can be carried out, and a process for participating in the transmission/reception process can be performed. Meanwhile, for the connected RRH, CSI-RS may be allocated independently.

FIG. 13 shows a table showing the connected RRH MAC control element.

In FIG. 13, the Ci field may include the RRH activation(1)/de-activation(0) command denoted by bitmap in the i-th global CSI-RS port. In case of UE activation, the UE can perform additional operations associated with the corresponding global CSI-RS port. In case of UE activation, the UE may perform additional measurement of CQI and PMI, and may participate in the transmission/reception processes.

Primary RRH Switching

In case of primary RRH switching, the global CSI-RS port number or bitmap of the primary RRH is transmitted to the MAC control element. FIG. 14 shows a table illustrating primary RRH switching MAC control elements.

In FIG. 14, the Ci field may include the RRH activation(1)/de-activation(0) command denoted by bitmap in the i-th global CSI-RS port.

Primary RRH change needs to be confirmed. This primary RRH change confirmation can be implemented by transmitting a primary RRH port number received by the UE, and this operation can be carried out by the MAC control element.

Associated RRH Set Update

If the primary RRH is changed, the associated RRH set can also be changed. Some methods for changing the associated RRH set can be used. There is a bitmap for the entire global CSI-RS port, a global CSI-RS port including an allocated bit is used as the associated RH, an additional bitmap for the associated RRH also exists, such that activation/deactivation can be notified using the additional bitmap.

All signaling processes may also be carried out using an ID of the RRH, or may be blind-processed by the global CSI-RS port number.

The following Table 2 shows the comparison result between a cell (S1, X2)-based handover and a C-RAN handover.

Table 2

TABLE 2 Cell based(S1, X2) HO C-RAN HO Inter-Cell Inter-cell IF based on S1 Implementation issue in Inter- and X2 is needed cell is controlled in MAC Signaling RRC message based MAC control element based reception ACK is needed (Activation/De-activation, and message transmission/ Primary node change) reception reception delay occurs. ACK is not always needed. RACH Full RACH procedure is Execution only in UL needed.-UL synchronization synchronization- Transmission/reception Dedicated RACH resource of Messages 3 and 4 are not needed HO Type Break before Make Make before Break Update Cell Radio Network C-RNTI (if required) Temporary Identifier(C-RNTI), logical channel ID(LCID), Security Context etc Etc Large delay in HO failure Legacy connection can be maintained in HO failure

Referring to Table 2, in case of the cell (S1, X2)—based handover technology, S1- or X2-based inter-cell IF is needed, signaling is based on RRC messages, reception ACK for message transmission is needed, and message transmission/reception delay may occur. The entire RACH procedure for handover is needed (UL synchronization and Dedicated RACH resources are needed). C-RNTI, LCID, Security Context, etc. are updated. In addition, a large delay occurs in handover failure.

In contrast, in case of the C-RAN based handover technology, activation/deactivation release and primary RRH change are signaled on the basis of the MAC control element, but it should be noted that reception ACK message of the signaled information are not always needed. The RACH procedure is performed only when UL synchronization is needed, and transmission/reception of Messages 3 and 4 in the RACH procedure is no longer required. C-RNTI may be updated as necessary. Although a handover failure occurs, the legacy connection can be maintained.

Exemplary embodiments described hereinbelow are combinations of elements and features of the present invention. The elements or features may be considered selective unless mentioned otherwise. Each element or feature may be practiced without being combined with other elements or features. Further, an embodiment of the present invention may be constructed by combining parts of the elements and/or features. Operation orders described in embodiments of the present invention may be rearranged. Some constructions of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions of another embodiment. Also, it will be obvious to those skilled in the art that claims that are not explicitly cited in the appended claims may be presented in combination as an exemplary embodiment of the present invention or included as a new claim by subsequent amendment after the application is filed.

It will be apparent to those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit and essential characteristics of the invention. Thus, the above embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the invention should be determined by reasonable interpretation of the appended claims and all changes which come within the equivalent scope of the invention are within the scope of the invention.

INDUSTRIAL APPLICABILITY

The method for enabling a UE to perform a handover in a Cloud Radio Access Network (C-RAN) system can be applied to various communication systems for industrial purposes. 

1. A method for performing handover by a user equipment (UE) in a Cloud Radio Access Network (C-RAN), the method comprising: measuring at least one candidate remote radio head (RRH); transmitting feedback information caused by the measurement to a primary RRH; and receiving information regarding a changed primary RRH from the primary RRH according to a result of the measurement, wherein the measurement is performed to discriminate each RRH on the basis of a channel state information-reference signal (CSI-RS) antenna port discriminated per the at least one candidate RRH.
 2. The method according to claim 1, further comprising: receiving list information of the at least one candidate RRH from a base station (BS) or the primary RRH.
 3. The method according to claim 1, wherein the at least one candidate RRH includes the primary RRH and an RRH communicating with the UE.
 4. The method according to claim 1, wherein the changed primary RRH corresponds to an RRH having a highest signal intensity from among the measurement result.
 5. The method according to claim 1, further comprising: transmitting a RACH to the changed primary RRH through RRH dedicated RACH resources.
 6. The method according to claim 1, further comprising: receiving information regarding a CSI-RS antenna port discriminated per RRH from a base station (BS).
 7. A user equipment (UE) for performing handover in a Cloud Radio Access Network (C-RAN) comprising: a receiver; a transmitter; and a processor, wherein the processor measures at least one candidate remote radio head (RRH), controls the transmitter to transmit feedback information caused by the measurement to a primary RRH, and controls the receiver to receive information regarding a changed primary RRH from the primary RRH according to a result of the measurement, and the processor performs the measurement to discriminate each RRH on the basis of a channel state information-reference signal (CSI-RS) antenna port discriminated per the at least one candidate RRH.
 8. The user equipment (UE) according to claim 7, wherein the processor receives list information of the at least one candidate RRH from a base station (BS) or the primary RRH.
 9. The user equipment (UE) according to claim 7, wherein the at least one candidate RRH includes the primary RRH and an RRH communicating with the UE.
 10. The user equipment (UE) according to claim 7, wherein the processor controls the transmitter to transmit a RACH to the changed primary RRH through RRH dedicated RACH resources.
 11. The user equipment (UE) according to claim 7, wherein the processor controls the receiver to receive information regarding a CSI-RS antenna port discriminated per RRH from a base station (BS).
 12. The user equipment (UE) according to claim 7, wherein the changed primary RRH corresponds to an RRH having the highest signal intensity from among the measurement result. 